It's the Romans' fault. Our modern calendar is loosely based on their old, confusing one. Though records on the Roman calendar are sparse and sketchy, legend has it that Romulus, the first king of Rome, devised a 10-month lunar calendar that began at the spring equinox in March and ended with December. It is unclear whether there were any official months between December and March, but it's likely they were left off because the wintertime wasn't important for the harvest.The second king of Rome, Numa Pompilius, decided to make the calendar more accurate by syncing it up with the actual lunar year—which is about 354 days long. Numa tacked on two months—January and February—after December to account for the new days.